The present invention relates to a method and apparatus for the control of an ignition system spark timing for a spark ignition internal combustion engine, and more particularly to a method and apparatus for the control of an ignition system spark timing during no load engine operation.
Recently, electronic spark timing controls have been developed which employ a processor, such as a microcomputer, to determine a spark timing (an advance value or a delayed value) based upon the engine operating parameters such as engine revolution speed and load. The electronic spark timing control features that, as compared to the mechanical advance unit like the conventional centrifugal spark advance mechanism or vacuum advance mechanism, there is substantially no limitation to the selection in the value of spark advance so that the optimum spark advance value can be set without any difficulty.
In the conventional system wherein a spark timing is set in accordance with the revolution speed and load (the load is detected with induction vacuum or intake air flow or the pulse width of a fuel injection pulse), since it is difficult to make a distinction between an idling or a coasting operation and a low load (light load) operation, if a value of spark advance is set large enough for low load operation like operation at a vehicle speed of 20 km/h, it results in a large value of a spark advance at the idling operation when the revolution speed and the load are substantially similar to those for a low load operation, thereby to cause instable idling or hunting in the revolution speed, while, if a relatively delayed value of spark advance is set for warm-up operation, it causes a deterioration in driveability.